U.S. Pat. No. 5,974,309 discloses a method and apparatus for monitoring cellular phone calls by a law enforcement agency. There, when a call is made to a monitored party, the law enforcement agency checks either a billing identification number, or a calling line identification information for recognizing the calling party.
Generally, when monitoring communications made or received by a monitored party in, for instance, a GSM system (Global System for Mobile Communications), or a packed switched network such as GPRS (general packet radio service) or UMTS (universal mobile telecommunication system), problems can occur in correctly tracing the party to be monitored. The monitoring (intercept) criteria known by the authorities is normally the mobile destination identifier (such as the phone number) of the monitored subscriber (MSISDN, subscriber's mobile destination identifier). The interception activation is therefore usually MSISDN-based. However, when criminals become aware of interception activities, they may for example use prepaid SIM (subscriber identity module) cards so that MSISDN-based interception becomes rather difficult. One might consider the use of IMEI (international mobile station equipment identity) as the interception activation criteria as it is more difficult and expensive for the monitored party to change their equipment instead of usually free SIM cards. In GSM, there exists an additional information IMSI (international mobile subscriber identity) that internally identifies the subscriber in a unique manner. However, the authority (normally a law enforcement agency but eventually also a licensed private agency or the like) activating the interception does not know IMSI so that MSISDN is more convenient in practical use.
Usually, MSISDN and IMSI have a one-to-one correspondence, with the mapping information being stored in a HLR (home location register). Although, theoretically, MSISDN can therefore be mapped to IMSI using the HLR information, this mapping necessitates time consuming processes and should thus be avoided if possible. Furthermore, the HLR is regularly updated so that it is laborious to maintain correct mapping information in the interception devices. Moreover, IMEI is no part of the HLR information because the subscriber's mapping to an IMEI is normally valid only for a short time, possibly only for one mobile connection.
There is therefore a need for a method and system which provide information useful for the monitoring, or which effect such a monitoring, in a reliable and uncomplicated manner.